Inflatable with tunnel hull, help please.

[Bos suzi]

Hi guys.

I am new to this forum and would really like some help from experienced inflatable boaters.

I have 3.3 m fully infalable with a 15 hp 2 stroke,

I recently installed small pontoons underneath (high jackers ) to create a tunnel hull, this was done professionally by one of the companies that specialize in tunnel hulls for racing.

I currently am strugling with handling issues,

It is realy difficult to to steer and have directional stability. If i am on the dam , it is realy hard to drive underneath a bridge as the boat flanks left and right and is extremely sensitive to any movement on the throttle in terms of steering.

My highjackers was around 0.8 bar, should it be pumped harder?

Should i avoid sitting on the oversized pontoons?

Should i try lowering the engine deeper in the water?

I am really worried , and would love just to have directional stability,

Can this be caused by running a trim which is too negative !?

It feels as if when i am on the plane, that the tunnel hull suddenly bites on the water trying to then change to direction of the boat.

Please help ,

Thank you.

[Bos suzi]

Anyone on here with a tunnel hull?

[rik_elliott]

Interesting. Any chance of a pic of the boat inflated, especially from underneath?
My old cat needed 14psi in the highjackers so 0.8 bar is in the right ballpark.

Tunnel hulls tend to need their engines run pretty shallow, at least on the zapcats so I generally wouldn't be lowering the engine, use a prop with some cup if you are ventilating.

We had to get the weight distributed carefully to get best performance - my mate at the front was effectively just mobile ballast to keep things balanced. If it's just you and you are sat on one side perhaps that is unbalancing the boat a little?

Can't help but wonder if your highjackers aren't quite straight or are moving about on the plane- hence the interest in seeing how they are attached and how straight they are!

[Bos suzi]

The front parts of the high jackers looks as if it bends slightly inward, like in the direction of the bow.

Please see pics, so the motor should be way higher than it is now hey?

At the moment i am chasing stability more than speed,



I tried correcting the negative trim on the motor, see timber wood in place for demonstration.

I was geting alot of kicked up water on the bow side but i guess that is just to much weight in front.



Thank you for the reply

[Landlockedpirate]

I have no experience of add on jackers, but even to my eye the engine looks far too deep. Is it a long shaft engine on a short shaft transom ? Even without the jackers this could give problems, once you put the additional tubes in there who knows what will happen to the handling characteristics of the hull.

The nearest we have to your set up in Europe is the Zodiac Futura, but the speed tubes are much smaller. The Futura runs fast a true with no handling problems till you are right on the performance edge.

I'm not suprised its got handling problems, suggest you need a short shaft motor and a trip back to the firm that fitted the jackers.

[Bos suzi]

My problem is my water inlet in underneath the anti ventilation plate, (the horisontal plate above the prop) .

So i am really scared for overheating of the placement if too high, or am i missing something guys?

[Bos suzi]

Does this look any better?

[bartiny]

From a non expert but keen enthusiast

I would say that the engine looks about right. if you can get the sib to ride on the tubes (enough speed) I do not think it should be any higher than it is.

Someone will be along that has a futura or the like and let you know 100%

This is in reply to your earlier engine height not the 1 above.

[captnjack]

These speed tubes look pretty big. Since they were professionally done, what did the installer say about a new engine height? Surely they might be able to offer some insights into their proper use.

I don't have any speed tubes but I suspect you need to:
1) sit in the absolute center of the boat
2) raise the engine up about 8 or 10cm
3) try not to go slow

[Exe treme]

Hi

A few thoughts for consideration beyond the engine position, but am I right in thinking the engine needs to be lower because the boat will be much higher in the water when planning?

I had a similar factory built set-up on a Lodestar Trimax 3D-V. The two main difference are:
1: The sponsons on the Lodestar are more inboard, so the main tubes still contact the water to provide lateral stability.
2: An inflatable keel runs the length of the hull to the transom, giving a much deeper V. This effectively produces a trimaran, offering better directional stability as the V cuts through the water.

As a comparison, your sponsons are nearer the bottom of the tubes, and your boat is much flatter. Therefore, when planing, the sponsons are the only part of your boat that can have contact with the water. There is no centre V to help directional stability, and there is no lateral stability from the main tubes, if the boat keels over beyond the sponsons.

This is only an idea, but perhaps the sponsons need to be moved further inboard, so the main tubes can help stabilise the lateral movement?

Here are some images of the Lodestar's design, including one showing engine height and tube position.

[captnjack]

Originally Posted by Exe treme

A few thoughts for consideration beyond the engine position, but am I right in thinking the engine needs to be lower because the boat will be much higher in the water when planning?

If the engine were the correct shaft length for the boat in the first place, yes. But it looks to be a long shaft engine on a short height transom.

[Landlockedpirate]

Originally Posted by Exe treme

Hi

A few thoughts for consideration beyond the engine position, but am I right in thinking the engine needs to be lower because the boat will be much higher in the water when planning?

I had a similar factory built set-up on a Lodestar Trimax 3D-V. The two main difference are:
1: The sponsons on the Lodestar are more inboard, so the main tubes still contact the water to provide lateral stability.
2: An inflatable keel runs the length of the hull to the transom, giving a much deeper V. This effectively produces a trimaran, offering better directional stability as the V cuts through the water.

As a comparison, your sponsons are nearer the bottom of the tubes, and your boat is much flatter. Therefore, when planing, the sponsons are the only part of your boat that can have contact with the water. There is no centre V to help directional stability, and there is no lateral stability from the main tubes, if the boat keels over beyond the sponsons.

This is only an idea, but perhaps the sponsons need to be moved further inboard, so the main tubes can help stabilise the lateral movement?

Here are some images of the Lodestar's design, including one showing engine height and tube position.

You might be on to something, the Futura also has the speed tubes more inboard and the keel is the same depth as the sponsons. Maybe sibs of this shape need to run like a trimaran to be stable ?

[Exe treme]

Originally Posted by captnjack

If the engine were the correct shaft length for the boat in the first place, yes. But it looks to be a long shaft engine on a short height transom.

Hi Captnjack

Understand that but there must be an additional 10cm (4") to the cav plate because of the sponsons. Not far of the difference between a short and long shaft?

[Bos suzi]

Hi guys, thanx for the imputs,
My engine is a short shaft, and the boat was set up for a shoft shaft, i cutted a groove into the transon because the manufacturer of the hi jackers said to i had to look at it to see if my motor isnt going to be too high,

The engine wasnt at the manufacturer while he was installimg the high jackers,

I agree with it needs something to improve the lateral flex, the guy told me to pump both the oversized really stif and the high jackers to about 1 bar to avoid flex otherwise it wont be as rigid, a inflatable keel might help too but doesnt make sense when trying to create air underneath the deck of the boat right?

I dont think the high jackers should be more inwards, personally i think its in the right position, it is really stable and easy to drive when the bow is lifted.As soon as the bow get lowered then it feels like the steering is exagerated it if steer left then it steers too left and the. I have to steer right and it steers too much to the right , this steering process is so harsh it feels like i get thrown off my balance.

I wonder if the start of the highjacers underneath the bow, is perhaps slightly started to early? Or perhaps nor exaclty straight, look at this comparison.

[Bos suzi]

Compare this photo to the one above?

Does my high jackers start perhaphs to easly and have a slight bend inwards ?

[Exe treme]

Hello again

The difference is that one has been designed to work like that, and the other has been coverted to look like that.

[rik_elliott]

Is your inflatable deck at full pressure? Zap cats have a steel transom, fibreglass nose and solid deck to stop the pontoons potentially heading off in different directions. If your deck flexes at all the tubes might go slightly toe-in or out leading to directional stability problems?

[Bos suzi]

Originally Posted by rik_elliott

Is your inflatable deck at full pressure? Zap cats have a steel transom, fibreglass nose and solid deck to stop the pontoons potentially heading off in different directions. If your deck flexes at all the tubes might go slightly toe-in or out leading to directional stability problems?

Its a wooden dect consisting of 4 wood pieces conected with aliminium insert pieces.

So it flexes a little bit, thank you for the input.


Trimming of the engine in n negative degrees, in other words the bow it to low so that it throws up alot of water, does that effect the directional stability?

[Locozodiac]

It's a costly experimentation, one thing is factory tested and built, other story add-on gadgets glued on standard sibs. Does sib have a central inflatable keeel, are all air chambers inflated o its factory recommended working psi ?

What does installer say about your extremely poor performance combo ? Is him a Pilatus type installer ? that is, sets and takes and washes hands afterwords. Installer should be responsable for his work, let him optimize combo performance for you...

Definitely Unhappy Boating

[captnjack]

Originally Posted by Bos suzi

Hi guys, thanx for the imputs,
My engine is a short shaft, and the boat was set up for a shoft shaft, i cutted a groove into the transon because the manufacturer of the hi jackers said to i had to look at it to see if my motor isnt going to be too high

It still seems a little high but ok.

Originally Posted by Bos suzi

I dont think the high jackers should be more inwards, personally i think its in the right position, it is really stable and easy to drive when the bow is lifted.As soon as the bow get lowered then it feels like the steering is exagerated it if steer left then it steers too left and the. I have to steer right and it steers too much to the right , this steering process is so harsh it feels like i get thrown off my balance.

You are bow steering. On a normal SIB this would happen with too much weight in the bow or the engine trimmed down too much. In your case its because of the speed tubes and the fact that you want to travel slow under bridges. You altered the boat so much it doesn't want to go slow. Its meant strictly for racing now with the bow off the water and only the rear portions of those speed tubes in contact with the water as you fly along at full throttle. If you want those (very large for the size and weight of the SIB) speed tubes then you are going to have to drive it much much faster to create the bow lift and give you directional stability. If you want to go slow and stable then cut off the speed tubes. You can't optimize the hull for both at the same time.